OBJECTIVE:: To analyze the anteroposterior displacement of the knee by means of stress radiography in individuals with unilateral anterior knee instability and relate to time of instability. METHODS:: Sixty individuals with intact knees (control group) and 125 patients with unilateral anterior instability (AI group) agreed to participate in the study. Gender, age, weight, height, age at injury, time between injury and testing, and surgical findings are studied. Both groups are submitted to anterior and posterior stress radiographies of both knees. Anterior (ADD) and posterior displacement difference (PDD) were calculated between sides. RESULTS:: In the control group ADD and PDD are in average, zero, whereas in the AI group ADD averaged 9.8mm and PDD, 1.92mm. Gender, age, weight, height, age at trauma and presence of menisci's lesions do not intervene in the values of ADD and PDD. Meniscal injuries increase with time. ADD and PDD do not relate with the presence or absence of associated menisci's lesions. The ADD and the PDD are related to each other and increase with time. CONCLUSION:: There is a permanent anterior subluxation of the injured knee that is related to the amount of anterior displacement that increases with time. Level of Evidence III, Study Types Case-control study.